Nb Sites Research Articles

Insight into the effect of Mo and Re on mechanical and thermodynamic properties of NbSi2 based silicide

The improvement of overall performance of high-temperature materials is still a big challenge. By means of first-principles approach, in this work, we systematically investigate the effect of transition metals (TM=Mo and Re) on the strength and high-temperature thermodynamic properties of NbSi2 with C40 structure. The phonon dispersions, Debye temperature and heat capacity of NbSi2 with alloying elements are calculated in detail. We predict that alloying elements of Mo and Re are thermodynamically and dynamically stable. These alloying elements prefer to occupy the Si site in comparison to Nb site. We further find that the compression strength of NbSi2 with alloying element of Re is up to 15.3% in comparison to perfect NbSi2. Importantly, alloying elements at Si site result in brittle-to-ductile transition for NbSi2. The heat capacity of NbSi2 with alloying elements is slightly larger than that of NbSi2.

Site occupancy of transition elements in C15 NbCr2 laves phase: A first-principles study

Using first-principles calculations, site occupancy behaviors of transition elements in C15 NbCr2 Laves phase are systematically investigated. Elements Y, Sc, Zr, Hf, Cd, Ta, Ti and Ag prefer to occupy the Nb site, and elements Zn, Pt, Re, Tc, Ir, V, Os, Rh, Ru, Ni, Co, Mn, Fe and Cu favor to occupy the Cr site; whereas elements Mo, W, Pd and Au have weak site preference for Cr or Nb site. The present calculations agree well with the available experimental and previously calculated results. It was found that the site occupancy behavior of transition elements in NbCr2 is mainly affected by the radii of transition elements. The present calculations also propose the correlation between the site preference energy and radii of transition elements.

Synthesis, structure and electrical properties of Mg-, Ni-codoped bismuth niobates

Mg-, Ni-codoped bismuth niobates Bi1.6Mg0.8-xNixNb1.6O7-δ (x = 0; 0.2; 0.4; 0.6; 0.8) were obtained by conventional solid-state reaction method. It was shown that the Mg atoms are distributed at the Nb sites while the Ni atoms are distributed over the Bi- and the Nb-sites, according to the results of comparison of pycnometric and X-ray density of the Bi1.6Mg0.4Ni0.4Nb1.6O7-δ pyrochlore. In this case, about 15–20% of the vacancies are formed at the Bi sites. The obtained compounds are stable up to their melting point based on the DSC analysis data. Real dielectric permittivity ε' of the Bi1.6Mg0.8-xNixNb1.6O7-δ samples decreases from 80 to 65 with the temperature decrease from 25 to 700 °C and practically does not depend on frequency in the range of 1–1000 kHz. Oxides Bi1.6Mg0.8-xNixNb1.6O7-δ behave like insulators up to 280 °C, their conductivity increases with temperature (Ea,dc ≈ 1.3 eV, dc) and with the Ni content at a given temperature.

Validity of Rigid-Band Approximation in the Study of Thermoelectric Properties of p-Type FeNbSb-Based Half-Heusler Compounds

Recently, we calculated the thermoelectric properties of p-type FeNbSb half-Heusler compounds by employing the rigid-band approximation (RBA) (Fang et al., RSC Adv 6:10507–10512, 2016). Traditionally, the RBA is used to understand and guide doping in semiconductors. It is therefore important to verify its reliability. To this end, we have investigated the validity of the RBA in heavily doped p-type FeNbSb by calculating the electronic structure and Seebeck coefficient of pure and Ti-, Zr-, Hf-, and Ce-doped FeNbSb using ab initio calculations. The results confirm that Ti, Zr, and Hf doping at Nb site shows rigid-band-like behavior, unlike Ce doping, which changes the density of states. We also calculated the electrical transport properties of the doped systems, indicating that the power factor of Ce-doped FeNbSb is lower than those of Ti-, Zr-, and Hf-doped FeNbSb.

Stable Metal-Organic Framework-Supported Niobium Catalysts.

Developing structurally well-defined, supported oxide catalysts remains a significant challenge. Here, we report the grafting of Nb(V) oxide sites onto the nodes of the Zr-based metal organic framework (MOF) NU-1000 as a stable, well-defined catalyst support. Nb(V) oxide was deposited with loadings up to 1.6 mmol/g via two postsynthetic methods: atomic layer deposition in a MOF, and solution-phase grafting in a MOF. Difference envelope density measurements indicated that the two synthetic methods resulted in different local structures of the Nb(V) ions within NU-1000. Despite their high Nb(V) loadings, which were equivalent to >60% surface coverage, nearly all Nb(V) sites of the MOF-supported catalysts were active sites for alkene epoxidation, as confirmed by phenylphosphonic acid titration. The MOF-supported catalysts were more selective than the control Nb-ZrO2 catalyst for cyclohexene epoxidation with aqueous H2O2 and were far more active on a gravimetric basis.

Origin of high thermoelectric performance of FeNb1-xZr/HfxSb1-ySny alloys: A first-principles study.

The previous experimental work showed that Hf- or Zr-doping has remarkably improved the thermoelectric performance of FeNbSb. Here, the first-principles method was used to explore the possible reason for such phenomenon. The substitution of X (Zr/Hf) atoms at Nb sites increases effective hole-pockets, total density of states near the Fermi level (EF), and hole mobility to largely enhance electrical conductivity. It is mainly due to the shifting the EF to lower energy and the nearest Fe atoms around X atoms supplying more d-states to hybrid with X d-states at the vicinity of the EF. Moreover, we find that the X atoms indirectly affect the charge distribution around Nb atoms via their nearest Fe atoms, resulting in the reduced energy difference in the valence band edge, contributing to enhanced Seebeck coefficients. In addition, the further Bader charge analysis shows that the reason of more holes by Hf-doping than Zr in the experiment is most likely derived from Hf atoms losing less electrons and the stronger hybridization between Hf atoms and their nearest Fe atoms. Furthermore, we predict that Hf/Sn co-doping may be an effective strategy to further optimize the thermoelectric performance of half-Heusler (HH) compounds.

(Invited) Correlating Ion Transport with Structure in Interstitial Containing Oxides:Potential for New Electrolytes?

Development of oxide ion conducting devices such as sensors, fuel cells and permeation membranes, has traditionally relied on the optimisation of materials with lattice vacancies through substitution, increasing the vacancy content and consequently conductivity. Whilst the ionic conductivity is a key parameter it is also essential to understand the surface exchange kinetics, stability and compatibility with other functional components. For many of the perovskite and fluorite based ion conductors the process of increasing vacancy content through substitution induces chemical instabilities, including cation segregation, that leads to reactivity and ultimately degradation in lifetime. In an effort to address some of these issues and to extend the compositional space available to materials developers we have considered the use of materials with inherent oxygen interstitial content. A range of materials have been considered, mainly based on the scheelite framework, in which significant interstitial content can be accomodated. Two main series of oxide niobates are the subject of this study: CeNbO4+d and LaNbO4. In the first series of oxides the oxidation of the Ce3+ species on increasing temperature induces ordering and results in a fast ion transport pathway through these phases. Unusually these materials are also structurally modulated, and anisotropic in nature. The oxidation of the Ce species also induces electronic transport. Substitution of the Ce species with divalent cations did not significantly affect the ion transport. For the second series of oxides we have focused on hexavalent substitution of the Nb site to incorporate interstitials, modulate the structure and reduce the electronic transport. Here we will discuss the potential of these modulated scheelite oxides as ion conductors, demonstrate effective electrolyte performance and correlate the ion transport with the structure of the materials, using a suite of advanced characterisation techniques.

Ab initio investigations of the structural, electronic, and thermoelectric properties of Fe2NbAl-based alloys

Ab initio calculations are performed to investigate the electronic structure of Fe2NbAl full-Heusler alloy as well as the non-stoichiometric Fe1.75X0.25NbAl and Fe2Y0.25Nb0.75Al (X, Y = Cr, Mn, Co) alloys. The thermoelectric properties of these alloys are computed using Boltzmann transport formalism. Fe2NbAl alloy is found to exhibit a semiconductor structure with an indirect band gap of 0.3 eV along the Γ–X high symmetry line. However, Fe1.75X0.25NbAl (X = Cr, Mn, Co) and Fe2Y0.25Nb0.75Al (Y = Cr, Co) are found to be metallic, whereas Fe2Mn0.25Nb0.75Al is semiconductor with a band gap of 0.2 eV at Γ-point. The p-type of Fe2NbAl alloy shows a maximum Seebeck coefficient of 220 μV/K at 800 K, whereas the n-type shows a peak of 280 μV/K at 150 K. The maximum power factor reaches 12 and 51 × 10−3 W/mK2 at hole concentrations of ~1.60 × 1021 and 1.45 × 10−3 cm−3 for the p-type material and about 18 and 42 × 10−3 W/mK2 at electron concentrations of ~1.65 × 1021 and 1.6 × 10−3 cm−3 for the n-type at 300 and 800 K, respectively. Alloying Fe2NbAl with Cr, Mn, and Co at Fe and Nb sites leads to an appreciable increase in the PF values of the non-stoichiometric alloys, which indicates higher efficiencies and potential thermoelectric applications.

No difference in local structure about a Zn dopant for congruent and stoichiometricLiNbO3

We compare extended x-ray absorption fine structure (EXAFS) data at the Zn K edge for a low concentration of Zn (0.7 mol%) in a stoichiometric crystal with that for higher Zn concentrations (nominally 5 and 9 mol%) in congruent ${\mathrm{LiNbO}}_{3}$ (LNO). Note that stoichiometric and congruent LNO have significantly different optical properties. We find no significant difference in the local structure about Zn out to 4 \AA{} for the two types of crystals and different dopant levels. Although some earlier theoretical models suggest a self-compensation model with 75% of Zn on a Li site and 25% Zn on Nb, we find no clear evidence for a significant fraction of Zn on the Nb site, and estimate at most 2%--3% of Zn might be ${\mathrm{Zn}}_{Nb}$.

Effect of Cu doping on room temperature ferromagnetic behavior of Mn doped LiNbO3 films

Cu and Mn co-doped LiNbO3 films were deposited on Si (111) substrates by rf-magnetron sputtering. XRD shows a randomly oriented polycrystalline R3C structure of LiNbO3 was formed in the films annealed at 1000°C for 1h in air. XPS and XAFS determine that Mn2+ substitutes on the Li site with a Li vacancy and Cu2+ substitutes on the Nb site with an oxygen vacancy in the LiNbO3 lattice. SQUID measurements indicate that all the films exhibit room temperature ferromagnetism, attributed to a strong d–d electron interaction between Mn and Nb and the bound magnetic polarons resulting from the oxygen vacancies. The saturated magnetization increases but the atom magnetic moment decreases with increasing Cu content in the films. The drop of the atom magnetic moment may arise from the antiferromagnetic coupling among adjacent Cu ions and an antiparallel configuration between Cu2+ ions and their trapped electrons.

Optimization of the thermoelectric properties of FeNbSb-based half-Heusler materials

FeNbSb-based half-Heusler compounds have recently been reported as promising materials for good high-temperature thermoelectric materials with a ZT > 1. Their electronic structure and thermoelectric properties are investigated based on a first-principles simulation and the semi-classical Boltzmann transport theory. The band structures show not only light and heavy bands but also high band degeneracy near the valence band maximum, which is beneficial for thermoelectric performance. The calculated Seebeck coefficients of p-type FeNbSb at high carrier concentrations exhibit the expected high values, which is consistent with experimental data. The evolution of the electrical conductivity and power factor with carrier concentration at different temperatures is investigated. Our results show that the thermoelectric performance of p-type FeNbSb can be improved by appropriate substitution; for example, by doping Hf on the Nb site, the maximum ZT of the p-type FeNb1−xHfxSb can reach ~1.5 at 1200 K. This study can provide some theoretical guidance for experimental research to improve the thermoelectric performance of FeNbSb-based half-Heusler compounds.

Raman spectra and ferromagnetism of nanocrystalline Fe-doped Li0.43Nb0.57O3+δ

Nanocrystalline undoped LiNbO3 and LiNbO3 doped with x% Fe (x=0.5, 1, 2, 3, 5) were synthesized via a combustion method. Fe-doped LiNbO3 with a 1mol% doping concentration exhibited a room-temperature ferromagnetism of 0.06emu/g. There was an abrupt change in properties when the doping concentration of Fe reached 2mol%, where the lattice contracted obviously and the saturation magnetization (Ms) increased an order of magnitude to 0.275emu/g; Ms slightly increased to its maximum value of 1.18emu/g when the doping concentration was further increased to 5mol%. Raman spectra showed that the substitution of Li by Fe occurred at small doping concentrations and the substitution of Nb at the Nb site occurred at higher doping concentrations. The results suggest that Fe3+ replaced NbLi4+ first and the weaker ferromagnetism is due to the minor fraction of NbLi4+ in LiNbO3. Then, Fe3+ substituted Li+, resulting in large lattice distortion and much stronger spin coupling of Fe–Nb. Finally, the excess Fe3+ started to replace Nb5+at the Nb sites, where the spin coupling of Fe–Nb is weaker than that at the Li site. An analysis of the experimental results suggests that the congruent Fe-doped LiNbO3 is a promising room-temperature single-phase multiferroic material.

First-principles study on the electronic structures and optical properties of Cu, Fe doped LiNbO_3 crystals

The binding energies, electronic structures and optical properties of LiNbO3 and Cu/Fe doped LiNbO3 crystals are investigated by first principles based on the density functional theory in this paper. The supersell structures of crystals are established each with 60 atoms, including five models: pure LiNbO3, LN1 (Cu2+ occupy Li+ site), LN2 (Fe3+ occupy Li+ site), LN3 (Cu2+ occupy Li+site and Fe3+ occupy Li+ site) and LN4 (Cu2+ occupy Li+ site and Fe3+ occupy Nb5+ site). The optimized results show that the total energies of all models can achieve certain stable values, which means that the models accord with the actual crystal structures. The impurity energy levels of Cu and Fe doped LiNbO3 crystals appear within the band gaps, which are contributed by Cu 3d orbital, Fe 3d orbital and O 2p orbital; in co-doped LiNbO3, Cu offers deep energy level and Fe offers shallow energy level within the band gaps. There are two wide absorption peaks appearing respectively at 445 nm and 630 nm in co-doped LiNbO3 crystal, which correspond to the electron transitions from Eg orbital of Cu to Nb 4d orbital and T2g orbital of Fe to Nb 4d orbital respectively; the absorption edge of Cu, Fe mono and co-doped LiNbO3 crystals are red-shift successively, which coincides with the variation of band gape. The light absorption intensity of co-doped LiNbO3 crystal is stronger than that of mono-doped LiNbO3 crystal. The co-doped sample light absorption property is related to Fe site occupation. In this paper, it is suggested that the co-doped sample with Fe at Nb site is more competitive than that with Fe at Li site in optical volume holographic storage applications, and that reducing properly [Fe2+]/[Fe3+] value may be conducible to the formation of this advantage.

Mn doping instigated multiferroicity and magneto-dielectric coupling in KNbO3

KNb1−xMnxO3 (0 ≤ x ≤ 0.07) samples were synthesized by a two-step process, ball milling of the precursors and then sintering subsequently. Mn doping at Nb site induces a weak ferromagnetism at room temperature and it has been explained with the help of bound magnetic polaron model. Magnetization values at 15 kOe initially increase followed by a decrease with increase in the Mn concentration. Two phase transitions—orthorhombic to tetragonal and tetragonal to cubic, above room temperature were analyzed from the differential scanning calorimetry and temperature dependent dielectric studies. It elucidates the effect of Mn concentration on phase transition temperatures. All the samples exhibit a peak behaviour in the dielectric constant Vs temperature plot at phase transition temperatures. The samples also exhibit leaky ferroelectric loops at room temperature. PUND measurement reveals the true ferroelectric polarization of the samples. The probable origin of magneto-dielectric effect is discussed based on the dielectric study in the presence of variable magnetic field at a frequency of 1 kHz.

Manufacture, electromagnetic properties and microstructure of an 18-filament jelly-roll Nb3Al superconducting wire with rapid heating and quenching heat-treatment

In this paper, we have reported the manufacture of a novel simple-structured jelly-roll Nb3Al precursor long wire, and its electromagnetic properties and microstructure with different rapid heating and quenching (RHQ) heat-treatments. By comparing three processing methods, it is found that the rolling and drawing (RD) method is more suitable to the fabrication of kilometer-length Nb3Al precursor wire without annealing. Using homemade RHQ equipment, we have successfully carried out RHQ heat-treatment of Nb3Al wire samples with various heating conditions. Based on magnetization and magnetoresistivity measurements, the onset superconducting transition temperature, Tc and upper critical field, Hc2 (0) of optimal Nb3Al wire reach 17.9 K–18.0 K and 29.7 T, respectively. Through microstructure and composition analysis, the Nb3Al superconductor in the optimal wire displays a typical ‘layer-to-layer’ structure, which comprises alternate crystalline Nb3Al and amorphous Nb layers, and the Nb3Al grains sizes are about 100 nm–300 nm. Furthermore, many small holes are dispersed in the Nb3Al superconductor due to the diffusion reaction from Al to Nb site. The work suggests the simple-structured Nb3Al precursor wire with RHQ heat-treatment is very promising for high-field application.

The stability of niobium-silica catalysts in repeated liquid-phase epoxidation tests: A comparative evaluation of in-framework and grafted mixed oxides

Two types of niobium-containing silica catalysts, (i) in-framework mixed oxide, obtained by co-precipitation type synthesis, and (ii) grafted niobium sites on silica supports, obtained via post-synthesis deposition from niobocene dichloride, were prepared. Both solids were tested in a series of five repeated batch-wise liquid-phase epoxidation tests of limonene with 50% aqueous hydrogen peroxide.Nb/SiO2 prepared by co-precipitation synthesis kept fully its epoxidation activity during the first two recycles, with a gradual decrease in activity in the following runs. Conversely, Nb/SiO2 prepared by deposition suffered from a more marked loss of activity since the first reuses. Most of the isolated Nb sites were maintained and no large Nb2O5 domains were formed during the recovery and regeneration tests. The distribution, geometry and coordination state of the Nb sites was therefore preserved, especially for the Nb/SiO2 sample prepared via co-precipitation. The presence of small Nb2O5 domains, even formed during repeated recycles, was not detrimental for the epoxidation reaction and, in general, Nb(V) sites inserted in a silica matrix proved to be a rather robust epoxidation catalyst for alkenes in the presence of aqueous hydrogen peroxide.

Investigation of defect structure of impurity-doped lithium niobate by combining thermodynamic constraints with lattice constant variations

The defect structures of impurity-doped congruent lithium niobates (c-LN) were determined for impurities with various valences, including divalent, trivalent, and tetravalent impurities, in a concentration range where antisite niobium (NbLi) exists. On the basis of the “Li site vacancy model,” six kinds of defect structures in impurity-doped c-LN are possible. Using thermodynamic constraints, these can be narrowed down to two kinds. The first structure is that in which impurities, vacancies and Nb exist as defects in the Li site and no defects exist in the Nb site (structure A), described as {[LiLi] 1-5x-jy[NbLi]x[MLi]y[VLi]4x+(j-1)y}[NbNb][OO] 3 (V: vacancy, M: impurity, j: valence of impurity, x, y: compositional variable (≠0), Li/Nb = congruent ratio). {[LiLi×] 1-5x-2y[NbLi••••]x[MLi•]y[VLi′]4x+y}[NbNb×][OO×] 3 is an example by the Kröger-Vink notation for divalent M. In the second structure, vacancies and Nb exist as defects in the Li site and impurities exist as defects in the Nb site (structure B), described as {[LiLi] 1-5x-(j-5)y[NbLi]x[VLi]4x+(j-5)y}{[NbNb] 1-y[MNb]y}[OO] 3. {[LiLi×] 1-5x+y[NbLi••••]x[VLi′]4x-y}{[NbNb×] 1-y[MNb′]y}[OO×] 3 is an example for tetravalent M. Since the relationship between impurity concentration and lattice constants for structures A and B differs, the defect structures can be differentiated by analyzing lattice constant variations as a function of impurity concentration. The results show that the defect structure of divalent and trivalent impurity-doped c-LN is structure A and that of tetravalent impurity-doped c-LN is structure B. The NbLi concentration increased with increasing tetravalent impurity concentration. In contrast, the NbLi concentration decreased with increasing divalent and trivalent impurities, leading to suppression of optical damage. The valence of an impurity determines whether the impurity is located in the Li site or Nb site in c-LN, consequently determining whether NbLi decreases or increases when the population of the impurity changes.

Ta-substituted SnNb2−xTaxO6 photocatalysts for hydrogen evolution under visible light irradiation

Ta-substituted SnNb2−xTaxO6 was successfully prepared via a solid-state reaction to study the effect of Ta insertion in Nb sites on the crystal structure, photophysical properties, and photocatalytic activities for hydrogen evolution.

Structural phase transformation and microwave dielectric studies of SmNb(1-x)(Si(1/2)Mo(1/2))(x)O(4) compounds with fergusonite structure.

Temperature- and composition-induced phase transition in SmNbO4 was studied by differential scanning calorimetry, Raman spectroscopy and high-temperature powder X-ray diffraction measurements. In situ X-ray diffraction studies revealed that SmNbO4 possesses a monoclinic fergusonite crystal structure at ambient temperature and transforms to a tetragonal scheelite structure above the transition temperature (To ≥ 800 °C). The second-order nature of this transition was confirmed by observing a linear relationship between the spontaneous strain (es) of SmNbO4 and the Landau order parameter (η) around the phase transition temperature. We stabilized this high-temperature tetragonal scheelite phase at ambient temperature by substituting Si(4+) and Mo(6+) into the Nb site of SmNbO4. The SmNb1-x(Si1/2Mo1/2)xO4 (x = 0.0-0.69) ceramic compositions were prepared by the conventional solid-state reaction method. Rietveld refinement was carried out on all the compositions to examine the phase purity, and the compositions where x < 0.06 all formed a monoclinic fergusonite structure (I2/a space group, Z = 2). Both the X-ray diffraction and Raman spectroscopy measurements revealed that increasing the concentration of x transformed the structure from monoclinic fergusonite to tetragonal scheelite (I41/a space group, Z = 4) at a critical concentration (xc). Both the monoclinic and tetragonal phases coexisted in the composition range of 0.06 ≤ x < xc. The Hakki-Coleman and reflection cavity techniques were used to measure the dielectric constant and quality factor of these stabilized phases, respectively. The temperature coefficient of the resonant frequency was measured by using an invar cavity attached to a programmable hot plate. The high-density samples possessed good microwave dielectric properties.

Syntheses, crystal structures, and magnetic properties of Mn–Nb and Co–Nb cyanido-bridged bimetallic assemblies

We report three-dimensional (3D) cyanido-bridged Mn–Nb and Co–Nb bimetal assemblies, MnII2[NbIV(CN)8]·(L)2·nH2O (1: L=5-aminopyrimidine, n=5; 2: L=5-methylpyrimidine, n=4), and CoII2[NbIV(CN)8] (4-pyridinealdoxime)8·2H2O (3). The single-crystal X-ray structural analyses show that 1 and 2 consist of isomorphic 3D cyanido-bridged network (monoclinic, P21/n) composed of square antiprism Nb sites and distorted octahedral Mn sites. Both 1 and 2 show ferrimagnetism with Curie temperatures (TC) of 32 and 34K, respectively, which is caused by the antiferromagnetic superexchange interaction between MnII (S=5/2) and NbIV (S=1/2) through the CN groups. 3 has a 3D cyanido-bridged network (tetragonal, I41/a) composed of a dodecahedral Nb site and axially distorted pseudo-octahedral Co site. This compound shows ferromagnetic ordering with a TC of 18K. Magnetization versus rotation angle of single crystal reveals that 3 has a large coercive field of 15000Oe as crystal is oriented to a-axis, which is due to the strong magnetic anisotropy on distorted CoII.